Predicting climate

As anyone who watches the weather knows, correctly predicting tomorrow's showers can be a tricky business. So how can we accurately talk about climate - the macrocosmic version of weather - in 50 or 100 years?

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After years of controversy, this year the jury is in: climate change is "unequivocal" and "very likely" caused by humanity, according to the summary of the fourth assessment report from the Intergovernmental Panel on Climate Change (IPCC).

The summary, released this February and snowballing calls for political action on climate change, is based on a compilation of research by 2500 scientists over six years. Their conclusions are that climate change is happening now and the effects are far reaching, even if greenhouse gas emissions miraculously ceased tomorrow.

During this century we can expect temperature rises of between 1.8 to 4.0°C and sea level rises of 18-59 centimetres. Positive feedback into the system is possible and could drastically alter the prognosis, for example if a reduction in rainfall caused the Amazon rainforest to dry up, or if the Greenland ice cap melted completely. In this case we could expect warming of around 6.5 degrees and sea levels rises of over seven metres, which would have catastrophic effects on how we live and our chances of survival. Billions of people would be affected and the world would face the greatest mass extinction since the dinosaurs.

We are in a fragile position to deal with climate change - there are more people than ever before and we have already brought our environment to the edge of habitability. With so much riding on the result of climate change, there's a global effort to pin down just what will happen, and when. But despite innovations in the way we understand how to predict climate, it's a dicey business - even a tiny alteration in the system can send results skyrocketing into unlikely scenarios.

The influence of chaos

The way we predict climate is by using models - equations that involve hundreds of variables, from temperature, to surface reflectivity, atmospheric composition and the brightness of the sun to name just the basics. The models used in the IPCC reports - atmosphere-ocean general circulation models or AOGCMs - are sensitive to chaos, the effect famously demonstrated by a butterfly that flaps its wings and creates a storm half a world away. Take one factor in the models and vary it by a tiny amount - say a difference of 0.01 degrees in temperature - and you can end up with a wildly different picture of how the Earth's climate might look in 100 years time.

To take chaos out of the climate system, scientists run models many times on supercomputers and look at the variability of the results - the results that sit in the most probable range are taken to be the most likely prediction.

Getting rid of the fudge factor

In the six years since the IPCC's last report, increasing computer capacity has helped improve models and there have also been improvements to the models themselves. Professor Andy Pitman from Sydney's Macquarie University, lead author of the chapter for model evaluation for the IPCC's assessment both this year and in 2001 says there has been dramatic change in our ability to understand and predict climate.

In 2001, most climate models needed what Pitman describes as a fudge factor - called flux adjustment. You needed to remove an amount of heat from the model's equations to stop the ocean warming results "drifting" - giving a result that was obviously far out from expected ocean temperatures.

"We can now simulate the way the ocean couples with atmosphere over thousands of years without the system drifting," says Pitman. We can accurately predict, for example, the continuing rise in sea levels beyond 2300 as the ocean slowly releases its stored-up heat.

How does a climate model work?

The core of climate models is built from Newton's laws of thermodynamics and equations that describe the motion of fluid around a sphere. The models incorporate hundreds of physical processes that influence climate; from the hemispheric scale, such as the El Nino Southern Oscillation, to the regional scale, like the mountains that trap rainfall near the coast.

Mapping all of the information that feeds into this system at every point on Earth is impractical, so climate models instead use an imaginary grid spaced over the Earth - horizontal grid points covering the latitude and longitude and vertical grid points covering the layers of the atmosphere. At each point is a set of data describing such things as composition of greenhouse gases, the temperature and the reflectivity of the surface (the albedo) due to cloud cover or ice.

Currently models have a resolution - grid spacing - of about a hundred kilometres, down from 500km resolutions used by models in the 2001 IPCC report. But there's a long way to go to fully resolve climate change, says CSIRO's Dr Tony Hirst, divisional coordinator of the Australian Community Climate Earth-System Simulator (ACCESS).

"Models currently give a pretty good simulation of seasonal change and atmosphere behaviours, however there is a lot of room for improvement, particularly at the regional scale," Hirst says.

Thunderstorms, for example affect climate on the scale of just a few kilometres. Other microcosmic influences include the dynamics of snow crystallisation in spring, eddies in the ocean, or the movement of moisture in the soil.

Survival of the fittest

To fit in the thousands of smaller influences on climate you need to dress the core with a giant set of accessories called parameterisations. These parameterisations are then tested around the world.

"We tend to find the models that are hopeless get kicked out of that [process]," says Pitman.

Pitman has evaluated many of the models used in the IPCC assessment, ranking them over Australia in terms of temperature and rainfall. The results aren't definitive; the best models fit about 90 per cent of the data. Using just these top models to predict climate presents us with an even grimmer picture of climate change, than the one we have already Pitman says.

"As we kick the weaker models out we get a better look at how the Australian climate will look. In term of temperature we find the maximum and minimum temperatures go up [by 0.5 to one degree Celsius]."

There's worse news in terms of drought, Pitman says: while current predictions show rainfall will increase over Australia away from the coast, kicking the weaker models out removes that increase in rainfall, even for the coast.

A cloudy crystal ball

Of the smaller climate process, those that confound climate models most are clouds. Scudding across the skies almost before we know they are there, clouds nevertheless play an important role in climate change, but whether they mitigate or enhance the effects of climate change is unknown.

It is almost impossible to account for cloud behaviour, because averaging the effect of clouds - the only way to put them into the climate modelling system, doesn't provide a clear picture of what their effect will be, says Hirst.

"Cloud convection is one of the key areas contributing to uncertainty in models," he says.

In 2007, we are standing at the edge of an abyss. Only for the last five years has the scientific world had the kind of resources it needs to find an answer to the complexities of climate. Before our future alters too drastically, we will no doubt hone our skills at understanding just exactly how the Earth will change. Keeping our climate from changing drastically is another problem.

Want to predict the climate future? Download the software climateprediction.net and your computer chugs away in the background testing and running climate models.